Testing of insulation.



S EVERSHED TESTING OF INSULATION. urucm'lon mip In. 21.1914.

1 212,467.. Pat nted Jan. 16,1917.

a SHEETS-SHEET 1.

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TESTING OF INSULATION.

nrmcmou man mm. 21. 1914.

I 1,319,467., Patentd Jan. 16,1917. I 8 SHEETS-8H5? 2.

v 'KITNESSES INVENTCR s. EV ERSHED. TESTING or INSULATION.

APPLI'CATION FILED IAI- 214 i914.

Patent/ad Jan. .16, 1917.

8 SHEETS-SHEET 3- INVENTUR; Syd ve s}; ed

WITNESSES (3.8. flaw/7a m mmmzvs S. EVERSHED.

TESTING OF INSULATION.

APPLICATION man MAR. 21, 1914.

1 212,467. Patented Jan. 16,1917.

8 SHEETS-SHEET 4.

4 IlIIIIIIIIIIIIlIlIlIIIIIlM Y 'Illllllllll IIIIIIIIIIIlllllilillllfl lllllll S. EVERSHED.

TESTlNG 0F INSULATION. APPLICATKON HLED MAR. 21. 1914.

1,212,457." Iatented 5211116, 1917.

8 SHEETS -SHEET 5.

WITNESSES v 5 a xvm gn 02 W 4 729 V3715 e C I I mmmzyzs.

S. EVE'RSHED. lESUNG 0F INSULAHON APPLICATION mm) MAR. 2x. @914.

Patented Jan, 16, 1%17.

8 SHEETS-SHEET 5.

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wmsss s s 3V Ins-had I ATTORNEYS w EJ W i S. EVERSHED.

TESTING OF INSULATION.

APPLICATION FILED MAR.21.1914.

Patented Jan. 16 1917;

8 SHEETS-SHEET I.

l NVENZT OR y e w Mm v AT TQRN S. EVERSHEDJ TESTING OF INSULATWN. aPPucAHou HLED MAR. 21, 19M.

mmgatsv Yatented Jan. 16,191

3 SHEETS-SHEET 8.

W mmm EYa thereby UNITEn STATES PATENT OFFICE.

SYDNEY TESTING Application filed Karen 21,

To all whom it may concern Be it known that I, SYDNEY Evnnsnnn, a subject of the King of Great Britain and Ireland, and residing at Acton Lane Works, Chiswick, London, W., England, have invented certain new and useful Improvements Relating to the Testing of Insulation, of which the following is a specification.

This invention relates to the testing of the insulation of electric systems and appliances and its object is to provide an improved method and means for ascertaining the condition of the insulation.

It has been known for many years that the resistance of insulation under ordinary working conditions is usually a good deal less at a high voltage than it is at low voltage, and for that reason it is customary .to make insulation tests at some pressure not less than the working voltage, a practice wh ch dates from the introduction of testing apparatus invented by 'me in or about the year 1889.

I have discovered by experimental research that one of the causes of the lower resistance at higher voltage is the presence or moisture in absorbent insulating materials in the form of drops and thin films which form leakage paths for electric current. When an insulator in that condition is subjected to electric pressure the propelling force known as electric endosmose drives water out of the drops into the films, increasing their thickness and therefore decreasing the resistance of the leakage paths. i

If the electric pressure is gradually increased tl1e insulation resistance of absorbent materials falls, rapidly at first but more and more slowly as higher pressures are attained and the relation of voltage to insulation resistance may be expressed by a'smooth curve which is convex to the base 1' ,from which resistance ordinates are me film sured, so long as the endosmose or efiect is the principal factor in determining its shape or law of curvature. Ultimately, it the increase in voltage is continued, the curve passes through a somewhat ill-defined point, or rather region. of inflection, and changing the direction of its curvature begins to bend downward toward zero resistance, the downward curvature rapidly increasing until actual breakdown occurs. The complete "characteristic curve, as it Specification of Letters Patent.

EVERSHED, OF GHISWICK, LONDON, "ENGLAND;

0F INSULATION.

Patented Jan. 16, 1917, 1914. Serial No. 826,372. I

may be called, consists therefore of two parts of opposite curvature joined at a point or region of inflection, the first part indicating the film effect already described, and the second part indicating the growtl'i of a dangerous mode of leakage which ultimately ends in a breakdown of the insulation. These two significant parts of the whole curve may be conveniently referred to as the film curve and the breakdown curve.

The law expressing the shape of the film curve is much the same for all absorbent materials, provided they do not contain more moisture than they can harbor in the form of drops and thin films; and the leakage which takes place under that condition is not of a dangerous character, since it merely consists in each a current which is well within its capacity as a conductor. Changes in the number of films acting in parallel do not afi'ectthe safety of the insulation, nor do they alter the law of the curve, although an increase or decrease in the number of film paths neces sarily lowers or raises the general level of the curve. Hence so long as the shape of the first part of the characteristic curve indicates conduction by films, the general level of the curve-high or low insulation resistance-is not a matter of much importance. On the other hand cessive amount of moisture, when it is in the form tions of water in bulk, is apt to produce a dangerous condition in consequence of the extensive electrolytic action which takes place under such conditions if the excess of water is not detected and removed before 7 particularly of local accumulathe conductors and insulating materials have;

been seriously damaged. But leakage con duction ofitliis kind follows Ohms law, that is to say, /the resistance does not vary with the voltage, and the characteristic curve would indicate this law by following a straight line parallel with the base line. Hence the shape of the first part of the characteristic curve enables safe and dangerous modes of leakage through damp insulating materials to be readily discriminated. Again, in those numerous cases in which the insulation is made up of two com ponents in series, one an absorbent material, and the other a non-absorbent substance which hnsa constant resistance, the shape of the first part of the characteristic curve will be the. r esultant at the two correspondfilm path carrying the presence of an ex modes of conduction. Hence the failure to curve begins to ,olru'un with for. either component will be accompanied by e ohsnge in the shape of the curve, which fuusuine' the shape of a normal film curve if the non-absorbent insulator breaks down, or will lose its film characteristic and loeoome u struigbt lino parallel to the base if the failure is in the-absorbent. insulator. Lastly, the region of inflection in the chap ucterlstic curve is'e highly significant fee,- ture, since it is there that u dungerousmiode leakage begins to be indicated Broadly ,is-peekingthe place where moisture conduction ceases to be the psi-amount influence, and the mu-dud us the limit for safe Working and so ng as the working voltage does not curry the ct srecteristic curve beyond that point the diction is not in danger." In these and other Ways um characteristic curve whether resistance ordinates or "coir duutimcokordinetes, provides for the first time go method-oi insulation. diagnosis, and

.- tbe present practice of testing insulutiozi eith vEntry therefore be usefully amplified in the lightof mydiscovery.

The invcntionfconsists in a method of diagnosis foruscertuining the condition of insulation by actual or implied reference to the Whole or any sigi'iiiicuut part of the characteristic curve deduced from tests oftbo resistance or conductance oftbe insulatime zit two or more suitably chosen and difforentlvoltages he invention also consists in the imrowed method for the testing of insulation ereiu described. hducli of the utility of my method of disgnosls consists in comparing, the low of shops of different characteristic curves obteined or from the insulation of an electric sysm or appliance at different times under or fromdifierent {I-ln currgdng out the invention the testing uppurutusis arranged and adapted to faciliate: such instance ordinates of all c which comparisons b reducing the re 7 srccteristic curves to numerical sceles which are convenient for comparison preferably 'so that all curves shall have one coiirdinete point in common, is preferably the initial point of the curve corresponding with the minimum test mg pressure. By this procedure all other points on the curves are expressed by their ratios or percentages with regard to the common polnt. To carry out this port of w the invention the resistance measuring instruncent or appliance is modified by the addition of a device adapted to vary the sensibility of theinstrument continuously or substantially so, over a considerable rouge so that uponmcking the first test to deter: nuns the initial or common point, the'indication of the instrument may be adjusted 5151331136 ordinates of the other bend downward may be re- I until itregistersone hundred or one thousand, or ten, or unity or some. other decimal multiple or sub-multiple, of a. hundred. When this adjustment has been made the retest-points m on ,the curve will evidently be indicated as percentages of the initial resistance: ordinate and the law of one curve may be readily compared with that of any other curve Without the necessity for any calculations, and in many cases without even going to the trouble of plotting the churacteristlc curves Again, when the object of the tests is to investigate some particular portion of a characteristic curve by the comparison of difi erences, as for example, when it is intended to ascertain the position of the point or region of inflection, then in carrying out the invention the testing apparatus is urrunged and adapted to facilitate the compurison of one difference with another, pretorsbly by means of adjustable rheostats (herein referred to us decrement rheo-- stats) adopted to give direct indications of the differences in the resistance (or conductunce) of the insulation, corresponding to successive increments of testin pressure. In carrying out the invention in e meesuring instrument or appliance may be of any own type suitable for the measurement of insulation resistance or insulation. conductuuce, but although the properties of insulstors may be expressed the testing apparatus is requlred to determine characteristic curves in terms of v01tego and resistance,

As a matter of convenience in testing the indicating instrument may be so arranged that insulation resistance or conductence may be measured in the customcry units whenever so desired without in any way limitin the power to vary the sensibility at Wil for the purpose of obtuining ordinates which are. readily compare le.

The necessary testing pressures ma be provided by a battery or alternatively by a dynamo or e magneto generator of direct current type, either hand driven or power driven as may be convenient, and adapted by means of multiple voltage devices to give the required series of testing pressures. A series of from tento twelve pressures, suitably graded, will generally enable "the complete characteristic curve to be investigated or drawn up to the breakdown point if the maximum pressure of the series is sufficient for the purpose. A series of six names? Y inflection and if the sole object is to determine the position ofthe point of inflection with respect to some assigned pressure the working voltage ofthe' system for example-then a series of three pressures will be enough. Any known principle either alone or in combination may be used for obtaining the multiple voltages required by my method of testing, as for example by a subdivided battery, or byvarying the speed of a dynamo, or by varying the induction or the number of effectiveturns of wire in the armature of a dynamo, or by a number of windings on a single armature each connected. to one of a corresponding number of commutators, or by a multiple armature each element having its appropriate winding.

In order that the invention may be better understood it will now be described with reference to the accompanying drawings of which:

Figure 1 is a diagram of a complete apparatus for investigating the whole or any part of the voltage-resistance curve of insulation; Fig. 2 is a modification ot the same in which the indicating ii'istrument .is an ohmmeter; Fig. 3 is another modification in which the eleetro-motire forces generated by three dynamo arniatures are combined to form aseries of testing pressures by means of a permutation switch; Fig. i is another modification of the same in which the speed of the generator which provides the testing current is varied by means of a change speed. gear in order to give a series of testing pressures; Figs. and 6 are modifications of the same, each adapted tor the in'vestlgation of a limited part of the voltage-resistance curve; Fi 7 illustrates apparatus for czu rying out my method arranged in a box for portability; Figs. 8, 9 and 10 shown modified form of'multiplef voltage generatoryin Figs. 3, 4, .5, 7 and 8 the armatures of the multiple voltage generator are alone shown, the field magnets and pole pieces being omitted in order to avoid confusion. v

" In Fig.1, E is a source of direct current adapted to give a suitable series of testing pressures. P. is: a multiple point switch udapted so as to apply any of the series of pressures to ...the insulation resistance V which is tobe investigated. G is an indi eatinginstrument adapted to show the value of the resistance .under test, and A and B are two parts of ,a variable shunt to the instrument coil C, by means of which the iniscries may be adjusted to some convenient. initial or. common point. T T are. the terminals whichseri e to connect the testing apparatus to the'insulation resistance which is to be tested. 4

In Fig. .1 the indicating instrument G is shown by way of example as a galvanonn' eter, a, being the galvanometer coil, (Z the index and F the scale. To obtain readings whlch are proportional to the resistance of the circuit the scale is shown divided and 'drawings. To provide for coarse and fine adjustments the shunt is divided into two parts R and '1' each sub-divided into a convenient number of sections and controlled by a multiple point switch. For convenience the resistance may be equal to a single: section of the larger resistance It. To maintain the resistance of the shunted galvanomcter at an approximately constant value the switch A is provided with a second switch arm which controls a series of make-up"resistances m m, and as a universal shunt gives the maximum resistance to the shunted galvanometer (i. c. to the combined galvanouietcr coil and the part of the resistance in parallel with it) when the current divides equally bet-ween the galvanometer coil and the shunt, the make-up resistance will bercduced to zero at that point. A resistance (Lot large valucisadded in series with the shunted galvanometcr in order to avoid excessive currents when the resistance VJ happens to be very low or'to break down under test. The value oi amay easily be made ten or even one hundred times as great as the resistance of the shunted galvanomcter so that a comparatively rough adjustment of the makeup resistances m m will be sufiicient.

In order that the galvanometer G may indicate in the same resistance units throughout the whole series of testing pressures, the voltage switch 1 is mechanically coupled to a n1ultiple-point shunt switch S which 0011- trols a number of resistances 71 7a,, etc, which serve as a series of shunts on the combined resistance formed by the resistance a and the. shunted galvanoineter. The value of each of these shunts is reckoned from the formula i I Resistance of shunt 7m: (a-+ r )/(nl),

where. y is the constant resistance of the shunted galvanometer and. n is the ratio of Fig. 1 the source of testing pressure E isshown by way of example as a battery subdivided into eight equal sections and controlled by the eight point switch P. Hence the shunt switch S also has eight positions and at the first point, corresponding with the testing pressure of asingle seetion'of the battery, the switch S stands at a blank contact so that the whole of the testing ourrent traversesthe circuit formed by the gal vanometerG andthe universal shunt A B. In this example the values of the shunts 0,, 10 7c, 70,, must evidently be' (a+g l-( +y), n w) -+9) It s desirable that the total resistance of the current circuit of the testing apparatus should be constant notwithstanding the effect of the shunts introduced by the switch S, and with this object another switch M is added to control a series of makeup resistances g.

In Fig. l the apparatus is shown with the initial voltage of the series applied for testing the insulation represented by WV, and the common point shunt A H is senior the minimum shunting effect. When so set the scale F may conveniently be arranged to read direct in megohms, and in that case the pointer d indicates 32 megohms. To bring the pointer to the required common point-in this example the scale division marked 100 would naturally be chosen as the initial or common point-the shunting effect is gradually increased by moving the 'contac arms'of the shunt A B in the. direc tion 0 adjustment by means of A and next making ac'ilitate the accurate setting of t iepointer to 'the desired initial reading, a small conthe common 'tinuous'lyedjustable rheostat may be intro-- duced as ndicated at N.

Having'set the galvanometer reading to e common. point the voltage is then in creased step by step by turning the voltage switch P in the direction of the arrow, the switches S and M being of course-carried Found from point to point at the same time.

i roceeding in this way, eight ordinates of the voltage resistance curve of the insulation W, (corresponding with the series of eight testing pressures) are rapidly obtained,'and ed to 100 then all the others are expressed as percentages and may be readily compared with other-ordinates obtained from other insulation or from the same insulation under different conditions With this apparatus the insulation resistance of W at any desired voltage of the series maybe obtained in megohms at any time by merely restoring point shuntA B- to the initial position of mini riumshunting effect.

In Figs. 2 and 2" the indicating instrument G is 'anchinnie'ter, c being (the current.

coil, and p the 'pressure'boilf' The coarse adjustment to a common'pointis efiected by a universal shunt A on the current coil, and the fine adjustment may be effected by simi-. lar means to those-shown in erably by forming a part of which is in serles with th the, resistance 6 e pressure C011 p) as an adjustable rheostat 3B. lfn'l ig. 2

the adjustments A and B have already ibeen the arrows first obtaining a rough vided by three may be driven by if the initial ordinate be adjust 1, or prefthe narrator efiected and tliepointer'd is shown standing at the scale division marked 1 preparatory' to raising the testing voltage step -by step to obtain the required series of resistgeared to or interlocked with the devices which eflect the changes in testing pressure, so that corresponding adjustments and voltage changes may be carried out either by themovementof one handle 01" by the to ordinated -move1nents of two interlocked handles. The measuring instrument mayconveniently have the scale so divided and figured that when the shunt for continuous it unnecessary to make ,-'"5 thestepby step adgustthe pressure is i d convenient to have the ad ustment devices it gives direct readings in ohms j or megohms when the instrument 1s arranged to give the maxlrnum sensibility, as

set for the minir In Fig. 3 the testing pressure E is proi dynamic or magneto armaaxle--as to constitute a, multiple-voltage testing generator.

' hand or by power, is adapted to run at a constant speed either by means of known means such tor. The three armature-s are so wound as to give three component pressures which when added algebraically, will give a numher-of suitable testing pressures. For example if the initial pressure is to be 6 bolts then armature X. should be wound to give 6 volts, and to obtain the greatest number of testing pressures from the three armatures, Y and Z must be woundto volts and 9e volts respectively.

The armatures X Y and Z are connected electrically to the three members P P P, of a triple permutation switch. Each of these members double-pole switch; P having 13 points to each pole, P havingfi points to each pole; and P, having'zpoints to each pole. Insulated slidingcontacts k i which serve to connect any pair of points with the: slip rings H L, are fixed to and-- rotate with the axles O O, and these axles are mechanically coupled so that'theyall rotate together in the same sense and thr' For example the three mutatioh switch This generator which give 36 turcs X Y Z, so coupled togetherflmechani- 'cally preierably by mounting them on one acentriiugal clutch or other as a constant speed mo-.

consists of a multiple .point he ualan les.- v nien ibers of the'iermay be mounted one above I er so that the insulated contacts h i may fixed to an axle which is common to these step by step changes the potential dif- :fcrcnce between the slip rings of the member P is as follows:zero, +36, +36, +38, ---3 2, -36, "3c, zero, zero, zero, +36, +36, +36; and the potential difference between the slip rings of the member P is zero for first 4 steps made by F and +96 during if: remainder of the 13 steps. These perions result in the algebraicaddition of component pressures generated by X Y and Z in 13 different as set forth in the following table:

a3 diner-once oi J mics on members. fl

" sum of the component i pressures.

l y l i l number.

i 3 a triple permutation switchis simple form in order but the on i. may be made to add t\ i, three, four or more ponent ssiurc giving a series of cal construction and. the disposition of the various parts of such switchesunay evidently be i widely without departing from the principle of algebraic addition. By way -& the permutation switch in Fi a in connection with a common pun adjuster ii ii and an ohmeter (l. The ii iation resistance il under test, is that rtiicen the :ommutator and windings of iaturc and the armatiu'c axle.

l the testing voltage is supplied from. 2; -31: sable speed gem "atOI' E which is driven from the wait 12 through a centrifugal constant speed clutch 1 -3 and (llilll speed gear .l'. Fim'cn pairs of gears are own suitably graduated as regards icinviif. ratioyand by way of example these gears are shown as actuated by means of sci-en friction clutches l. 2. 1-3 ii of the mag: c I hen-c a? trailed my multiple point switch l ed to the exciting coils of the y which several component presll) or more voltage steps. The mechaniv by means of slip rings and brushes as indicated by 9 and 10. Current for the magnetic clutches may be derived from any suitable supply, and in the absence of any other available source a small dynamo may be provided for the purpose and driven at a constant speed, preferably by mounting its armature 8 upon the constant speed shaft of the change-speedgear. The connections of the galvanometer indicated correspond to Fig. l.

Other known forms of change-speed gear may be employed preferably of, the types which are actuated by friction clutches, and when the indicating instrument G is a galvanometer, the mechanism which actuates the clutches should be mechanically connected to or interlocked with the voltage I teristic curve a comparison of differences may be made. For example, to ascertain whether at any assigned voltage lhxthe characteristic curve has reached or passed bcyond the point or region of inflection, it is sufiicieut to test the insulation at three pressures namely Ee, E, and E-l-eabta and to ascertain the corresponding resistance values, say 0;, Z), and c, respectivcly- Then it is found that 7)(: does not exceed a the curve has not been carried beyond. point of inflection by subjecting the insu tion to the pressure E. Those who are acquainted with the mathematical theory of curves will ungerstand that thisprocess of comparison is ased upon the fact that at a point of inflection dr/(ZE has its minimum value and (Pr/(ZIP changes its sign on passing through zero. In most characteristic curves of insulation the point of inflection is somewhat ill-defined, being a region of inflection rather than a point, and it is therefore generally impracticable to make the necessary test with small increments of pressure. In practice it will often be necessary to make the difference 0 volts as much as one fifth orcven one fourth the raise of the as signed pressure E, in order to insure a measurable difference in the corresponding re sistance values. Hence the differential sensibility of the measuring instrument should be asgreat as possible, and for carrying out the process of comparison of difi'crences a lVhcatstonc bridge or a dilforential galvanon'xcter ismore suitable than a "alvanomcter to indicate the amount oi man resistances be widely different msg ni' tv des may be, balanced. The resistcncein the third arm, corresponding with thenct" justcble arm of on ordinary bridge, is divided into vtwo sections each of. which is separately adjustable and so arranged e regards the marking and figurin oi the resist-- snce values that the scale reading or figures indicate the decrement or amount of resistunce withdrawn or subtracted from the total amount initially contained in the third arms-the inverse of the ordinary arrange ment of c rheostct, which would. be figured resistance added. In Figs. 5 and 5 0., enclb are the ratio arms of a. Wwatstone bridge of which the rheostat V and decrementrheostcts D and D form the adjustable arm. The indicating instrument G may be e sensitive gulve .nometer of any known type which is suitable for use with'e bridge. The testing pres sures may be supplied from any convenient source; by way of example at two component generator having two armetures X and Y is shown, and their component pressures'ere algebraically added by switch having two mem I end P When this modification is intended for the investigation of the shape of the voltageresistence curve of the insulation of some electrical system at or about the working voltage then the armature Y may be wound to generate a pressure equal to the working voltage, say Evolts, and the armature X may be Wound to generate a fraction of that pressure, say 6 volts. For example if E, the

working voltage. be 500 volts then 6 might very conveniently be made equal to 100 volts;

In Fig. 5 shown set tothe second voltage point which gives Ee volts-us the testing pressure.

.to the third At this pressure the bridge has been balanced by first adjusting the ratio arm 6 so that it bears a, sufficiently small ratio to a, and then obtaining an acurute bulunce by ad justing the rheostut V-the decrement switches having been initially set so that each stands at zero scele reading. corre sponding with maximum resistance. The permutation switch would next he moved point giving a. testing pressure of E volts. If the insulation resistance' xV, which is under test, is that of an absorbent insulator containing moisture then the raisin of the voltzige from E@ toll) volts will bring about a decrease in resistance end the bridge will be'thrown out of balance. To restore the bale'nce the control arm of the decrement rheo'stat D must be moved in the direction of the arrow until the gclve nometer G once more indicates a balance. The testing pressure would now be raised another step to E+c volts by moving the permutation switch to the fourth point, thus bringing about a further decrease in the resistence oil Vii c gern utiition. hers I the permutation switch .is'

the helcnceof the biidgc.

moved 7 J the balance was cgcm restored. lnsgiec i 3 tion of the readings of thedecremcnt chilie end; e-gein' unsettl ng The contest cm of the decrement rlieostet D would now in the direction of the crrow untilf.

state will now indicate ct oncewhich see.

reinent'wcs the greater.

If the, first deck mznent isgrceter'thcn the second new oh viouslv thcresistnncc curve is convex the base line at the pressure Evolts. in the ill other hand if the decrement is smellcr than the happen second then the curve is concave to the base line, and if the two decrements to be equal then the curve enn st? there i321 sensibly e strtight line between the pressures E e end point hetween. those two pressures. In Fig. 5 the final position oi the contact urine-lee shown by the dotted lines) indicates a, decrement of about 22 and c ment of about 1 from whicin it would he inferred that ct the "working volts the convci: to the base line and was pressure still some consideriulile distance on the safe side of the point or region of lili'l@fil;l0n..

This difiei' enticl processmuy be cxtenderi'so us thus certain any desired nuinberof decrements of resistance by" making a. number of increments of testing pressure, the resistance in curve of insulation resistance Wes Elwyn)? there must n, omtof inflection on the curve at some second deem the third arm of the bridge being snbdividerl into sections and such section figured to indicstedecrcnient of resistance.

Fig. 6 illustrates u modification in which a diflferenticl gslvsnometer is' substituted for the "Wheetstone bridge which has just been described. The indicating instrument G is u gulvsnometer huvin two clifiei'en tic-11y wound-coils G and which curry the leakage current and the balancing can to obtain the" rent respectively. In order maximum sensihiiity the leakcge coil 6. may be wound with the greatest possible number of turns of fine wir convenience the resistance of the balancing circuit shouidfloe very small compared with the values of insulation resistance W which are likely to be measured the balancing coil (l should be wound with o relatively small number of turns the required number of separate.

6, and since for.

of wire. If a further ale reduction in the relative. effect of the hol uncing coil is reguired it may be obtnined by means of c shunt A. connected; sieross its terminals. By wey of example an adjustuble universul shunt is shown.

The balancing; circuit is completed through en adjustable rheostet V and two decrement rheostnts D and D The way of obtaining the rcspondiungv with voltage increments is of course analogous to that which has just been described invconnection with Fig. 5.

resistance decr ments cor then increased The decrement rheostats having been set to zero scale' reading, the lowest of the three testing pressures is applied by means of the multiple voltage switch P, and a balance is effected by adjusting the rheostat V a preliminary coarse adjustment being made. if required by means of the adjustable shunt A. The testing pressure is one step at a time and the consequent reductions in the insulation resistance are balanced oneatter another by means of the decrement rheostats.

In Fig. 6 the decrement rheostats are so modified that the two decrements are indicated in a scale which is common to both. By way of example the second decrement is shownlarger than the first, this indicating that the insulation curve is concave to the base line, and that at the middle voltage the curve has been carried beyond the point of inflection. In Fig. G a fixed-coil galvanometcr is shown, but any sensitive type of differential indicating instrument may be used.

For portability and convenience in use, a multiple voltage generator and a measuring instrument with or without any of the vari-" ous devices herein described for facilitating the process of testing by my method, may be mounted in one box or case or the generator and an of the devices may be so mounted or again the instrument and any of the devices may be similarly mounted. One such arrangemcnt is illustrated in plan in. Fig. 7. The testing generator l shown in this drawing is of the three-component type and is arranged for hand driving I by means of a winch handle 15. The necessary high speed is obtained by suitable gearing 14 interposed between the winch axle and the axle 12. One end of the axle 12 carries the friction drum of a constant speed centrifugal clutch 13, by which means the three armatures X Y and Z are driven at a constant speed whenever the slipping speed of the clutch 13 is exceeded. Since the armaturcs Y and X are only required to gencrate'onethird and one;ninth the pressuregenerated by Z they may be suitably graduated in size, and to avoid a long span between the bearings oi the armaturev axle, the armature X may be overhung outside the. end bearing. carrying box 16 may be so proportioned as to provide space for the indicating instrument G at one end, and for the common point adjusters A l3, and the permutation switch P, alongside the multiple voltage cncrator and the indicating instrument.

The methods of obtaining a suitable series of testing pressures described in connection with Figs. 1 to 7 may be modified in various ways. For example when the. armature of the testing gcncratoris wound with a number of separate coils each connected to one of a number of distinct two part commu- The tators (an arrangement commonly used in generators for insulation testing) then a series of voltagesmay be readily obtained from the one armature by connecting a number of tappings from each coilto a multiple point switch, the switch being mounted upon the armature axle and operated by suitable means' A multiple voltage armature conmultiple point switch P as indicated in Fig. i

At suitably graded intervals the course of the winding 29, ta'ppings are brought to the contact studs of the switch P so that the whole or any prearranged portion of the winding may be connected to the'commutator. In this way each armature coil may be made to provide a series of pressures, and by connecting thebrushes of the commutator-s 30, 31, 32 and 33 in series, as indicated iii Fig. 8, the separate pressures of the four coils are added together to provide the testing voltage.

There being four armature coils, the complete switchP in Fig. 8 will be composed of four multiple point switches as shown in end-elevation in Fig. 9. The four insulated contact arms 17 are mounted upon a sleeve 18 which is free to rotate on the armature axle 19. Guide pins 22, which project in side an extension of the switch sleeve 18, engage in spiral slots 21 formed on the outside of an operating sleeve 20. The oprating sleeve can slide to and fro on the armature axle but is prevented from turning around on it by a feather 23 which engages 1n a keyway inside the sleeve. The operating sleeve 20 can be set at will to any one of a number of positions along the armatnreaxle by means of the fork lever 24 which can be retained in the. desired position by a notched sector 25 or equivalent device. By these means, or their equivalent, the longitudinal motion given to the sleeve-9.0 is converted into a rotational movement of the contact arms of the switch P and hence the switch can be set to any desired position notwithstanding the rotation of the armature.

In Figs. 8, 9 and 10, 6-way switches are shown, by way of example, corresponding to a series of six testingpressnres. Similar switches may be made for a larger number of ways provided the overall diameterof the complete switch can be sufficiently enla rged to admit of the necessary addition to the. number of contact studs.

The invention is not confined to the particular arrangements described, but may be without depart modified in' various ways a ntense ing from my. underlying principle of diagnosis. For example if it is, desired to carry out myj'invention by means of voltage-conductancecurves instead of voltagereslstance curves the measuring instruments would be modified to indicate conductance units. In the case oil a galvancmeter the modification would consist merely in dividing the scale in direct proportion to the current; in the case of an ohmmeter the scale would be divided in inverse proportion to the resistance and figured in reciprocal ohms or other units of conductance. A. Wheatstone bridge or a differential galvanometer would need no modification except in the rheostats in the adjustable third arm, which would be composed of a number of unit resistances adapted so that any desired number could be connected up in parallel to give any total value of conductance. These and other similar inversions either necessary or convenient for the purpose of converting resistance measuring apparatus into conductance measuring apparatus will be readily understood by those who are conversant with the art of electrical measurement.

It is to be understood that in making measurements to determine two or more points on a characteristic curve the tests must proceed by ascending voltages from the lowest required testing pressure upward step by step to the highest pressure of the required series, in order that hysteresis effects, which are a marked'feature of con- (luction through absorbent insulators, may be avoided.

Having now described my invention what I claim as new and desire to secure by Letter Patent is l. A method for the diagnosis of the insulation in installed systems and appliances which consists in applying to the insulation a plurality of suitable and different voltages and measuring the resistances or quantities related thereto at each voltage.

2. A method for the diagnosis of insulation which consists in applying to the insulation several. diilerent voltages and measuring the resistances or quantities related thereto at each voltage the voltages being so chosen that the location of the point or re gion of inflection on the characteristic curve is ascertained with regard to an assigned voltage.

3. A method for the diagnosis of insulation which consists in applying to the insulation several different voltages and measuring the resistances or quantities related thereto at each voltage, the voltages being so' chosen that the point or region of inflexion on a curve connecting resistance or conductance and voltage applied is included,

consists in c nnecting the insulator in cir- 'cuit with an adjustable source of electromotive force and a galvandmeter oi adjustable sensibility; adjusting the galvanometer deflection to a convenient initial. reading upon a scale divided in units of resistance (or' conductance); successively adjusting the eleotromotive force to a series of dillerent voltages and simultaneously adjusting the galvanomcter so that throughout the saidseries it shall indicate, by units of the same magnitude, the values of the insulation resistance or conductance corresponding to the series of electroinotive forces.

5.. A method of testing insulation which consists in connecting the insulator in circuit with an adjustable source of electro motive force and a galvanometer of adjust able sensibility; adjusting the galvanome'ter deflection to a convenient initial reading upon a scale divided in units of resistance (or conductance); successively adjusting the electi'omotive force to a series of dilierent voltages and simultaneously adjusting the galvanometer so that throughout the said series itshall indicat by units ofvthe same magnitude, the values of the insulation resistance or conductance corresponding to the series of electromotive forces, and

in terms of the initial reading.

'6. A method oftesting insulation which consists in connecting the insulator in circuit with an adjustable source of electromotive force and an ohmmeter of adjustable sensibility; adjusting the ohmmeter deflecion to a convenient initial reading; successively adjusting the electromotive force to a series of different voltages and obtaining a corresponding series of values of the insulation resistance (or conductance) upon the ohmmeter scale. 1

7. A method of testing insulation which consists in connecting the insulator in circuit with an adjustable source of electromotive force and an ohmmeter of adjustable sensibility; adjusting the ohmmeter deflection to a convenient initial reading; successively adjusting the electromotive force to a series of different voltages and obtaining a corresponding series of values of the insulation resistance (or conductance) upon the ohnnneter scale, in terms of the initial reading. i

8, A method of testing insulation which consists in connectingihe insulator in air .cuit with an adjustable source of electrothe galvanometer by variable resistances to an initial balance; successively changing the parisonof the aforesaid resistances or qnantitles" related thereto the condition of the insulation is ascertained.

11. The improvement in the art of ascertaining the condition of insulation which consists in applying to the insulation a series of two or morevoltages in ascending order of magnitude suitably graded with regard to the working voltage and measuring the insulation resistance or quantities related thereto at each applied voltage.

In testimony whereof, I have ailixed my signature in presence of two Witnesses.

SYDNEY EVERSHED. Witnesses G. -B.- Roman, -O. SCHNEIDER. 

